David W.E. Roberts wrote:

However maximum stress is when the bath is filled to the overflow with one
(or if you are lucky) two people in it.

True, true.

[Lit candles for effect are considered of negligible additional weight.]

You romantic!

Therefore logically you should apply silicone sealant with the bath filled
and at least one, preferably two (British Standard) people in it.

They don't have to be British Standard. Assuming it's your bath, they
just have to be you and your chosen fellow bather. And they come in
lots of non-standard sizes, and in various international flavours.

Conclusions:

(1) I am not sure the guy who fitted the new bath in our bathroom a few
years back knew the correct way to do things.

Or he thought it would be a bit odd for him to ask your wife to get in
the bath with him, as a rough guide!

"It was for professional purposes only!

--
Ben Blaney
GSF1200 VFR800 CBR600 CD200
"We stopped only for fuel"